Cryogenic distillation is currently the preferred process for producing high purity oxygen (&gt;95%) in large scale plants (50-2000 ton/day). However, contaminants in the compressed air feed, i.e., water, carbon dioxide and trace hydrocarbons, must be removed before conducting the distillation process in order to prevent blocking of heat exchangers or distillation equipment and buildup of hazardous concentrations of hydrocarbons in the distillation column sump. Reversing heat exchangers are commonly employed to remove contaminants in the front end of the cryogenic plant wherein such contaminants are condensed in the exchanger passages and then removed with a waste gas stream. Alternately, absorbent beds containing zeolites or alumina which require periodic regeneration are used to absorb such contaminants. In addition, hydrocarbons must often be removed from the liquid oxygen sump by using an adsorbent such as silica gel. These methods lead to increased capital costs and inefficiencies in the overall separation processes.
Alternate methods for recovering oxygen from an oxygen-containing gaseous mixture include vacuum swing adsorption (VSA) and pressure swing adsorption (PSA) processes which employ selective adsorption of various components instead of conventional cryogenic steps to separate the mixture. As in the case of cryogenic processes, one or more of carbon dioxide, water and hydrocarbons must be separated from the feedstock prior to running the process to avoid deleterious interaction with the adsorbents.
Typical processes for removing carbon dioxide and water from an oxygen-containing gaseous mixture employ an adsorbent or desiccant and are capable of removing water vapor to very low levels, often to a dew point of less than -50.degree. F. These processes possess a drawback in that the adsorbent bed must be regenerated, usually by purging the adsorbent bed with a low pressure dry waste gas or by using some portion of the product stream if a suitable waste gas stream is not available. Consequently, these systems are operated in a cyclic manner requiring duplication of equipment, operation of automated, timed switching valves and separate heater devices. An unavoidable loss of the gaseous feed often occurs during regeneration of the adsorbent.
U.S. Pat. No. 5,108,465 discloses a process for separating oxygen from an oxygen-containing gaseous mixture which comprises contacting the oxygen-containing gaseous mixture with a membrane which is impermeable to gas yet which is capable of conducting electrons and oxygen ions. The membranes are formed from a ceramic material selected from the group consisting of BaFe.sub.0.5 Co.sub.0.5 YO.sub.3 ; yellow lead oxide; ThO.sub.2 ; Sm.sub.2 O.sub.3 -doped ThO.sub.2 ; MoO.sub.3 -doped Bi.sub.2 O.sub.3 ; Er.sub.2 O.sub.3 -doped Bi.sub.2 O.sub.3 ; Gd.sub.2 Zr.sub.2 O.sub.7 ; CaTi.sub.1-x M.sub.x O.sub.3-.alpha. wherein M is Fe, Co or Ni, x is 0-0.5 and .alpha. is 0-0.5; SrCeO.sub.3 ; YBa.sub.2 Cu.sub.3 O.sub.7-.beta. wherein .beta. is 0-1 and (VO).sub.2 P.sub.2 O.sub.7.
The ceramic materials disclosed in U.S. Pat. No. 5,108,465 comprise ionically conductive materials, which are commonly used in fabricating solid oxide fuel cell components, and superconducting materials. For example, YBa.sub.2 Cu.sub.3 O.sub.7-.beta. is an ionically conductive superconducting material. However, barium-containing ceramic materials are well known to be adversely affected by the presence of carbon dioxide. For example, literature references teach that water and carbon dioxide will irreversibly degrade YBa.sub.2 Cu.sub.3 O.sub.7 destroying superconductivity properties of the material upon contact with carbon dioxide at temperatures greater than about 400.degree. C. This phenomena is discussed in numerous articles including those by E.A. Cooper et al., J. Mater. Res. 6 (1991) 1393 and Y. Gao et al., J. Mater. Res 5 (1990) 1363. Therefore, gas separation membranes formed from ceramic materials which are adversely affected by carbon dioxide, water or hydrocarbons have only been used to separate oxygen from oxygen-containing gaseous mixtures which are substantially free of the same. Consequently, one of ordinary skill in the art would be lead to believe that such processes would only be of only limited value in separating oxygen from atmospheric air which contains appreciable amounts of water, carbon dioxide and hydrocarbons.
Considerable effort is being expended in developing an oxygen recovery processes wherein the feedstock does not have to be pretreated to remove carbon dioxide, water or volatile hydrocarbons prior to conducting the separation. Moreover, improved processes are being sought wherein the ceramic membranes used to effect separation of oxygen from oxygen-containing gaseous mixtures can be regenerated periodically, if required, without stopping the process.